Many chemical products are produced industrially in processes operated continuously or semicontinuously. Here, one or more starting materials are processed further in a chemical reaction to form other materials. One or more of the materials produced in this way can in turn go into a subsequent second chemical reaction in which the desired end product is finally produced. Here, continuously means that materials are fed without interruption to the chemical reactions and that the chemical reactions produce reaction products (in the subsequent process the starting materials or the intermediates) without interruption. “Without interruption” here refers to the periods of time in which the reaction actually takes place and does not exclude the possibility of a reaction being interrupted, for example as a result of a maintenance shutdown. In the following, the substances fed into the production processes are referred to as starting materials, abbreviated to “E”. The products flowing out from the outlet of the plant are referred to as output materials, abbreviated to “A”. The intermediates are denoted by “Z” by way of abbreviation.
In such nested processes, coproducts are inevitably also formed because of the prescribed stoichiometry of the underlying reaction equation. In industrial processes, efforts are made for reasons of economics and environmental protection to reuse coproducts formed to the greatest possible extent. For example, hydrogen chloride is obtained as coproduct in many chemical processes such as the preparation of isocyanates by phosgenation of the corresponding amine compounds and can be used again, for example after oxidation to chlorine.
Such coproducts can likewise be formed in multistage reactions, and there are cases in which the coproduct of a subsequent reaction can be fed as starting material to the first chemical reaction, optionally after prior treatment. Such cases include the preparation of dinitrotoluene by a first chemical reaction, viz. the nitration of toluene to form nitrotoluene, followed by a second chemical reaction, viz. the nitration of nitrotoluene to form dinitrotoluene. The nitration is usually carried out by means of a mixture of nitric acid and sulfuric acid, giving an acid phase (“used acid”) which is diluted by the coproduct of the nitration, namely water. The second reaction to dinitrotoluene requires a higher sulfuric acid concentration than the first reaction to form nitrotoluene. The concentrated sulfuric acid is therefore usually introduced into this second reaction step, this is separated off after the reaction is complete and sulfuric acid which has been diluted by the reaction is introduced into the first reaction step. Here too, water is formed as coproduct and dilutes the sulfuric acid further. In the first reaction, too, the sulfuric acid is separated off. This is usually concentrated up and can subsequently be used again in the second reaction stage.
Such a process is described below in abstracted form in FIG. 1; the materials Z1 and A2 can be considered to be coproducts since they are obtained in a predetermined particular ratio to one another because of the nature of the reaction which proceeds. The coupling of these two chemical reactions in this way has many economic and ecological advantages since the amount of waste streams obtained is minimized in this mode of operation. However, such coupling of two chemical reactions also presents challenges.
The mass flows of all materials fed to the chemical reactions (hereinafter also referred to in general terms as materials) have to be matched precisely to one another. This has hitherto been achieved by setting down the intended flow rate Fi,int or the actual flow rate Fi,act of a selected material and regulating the mass flows of the other materials to suitable intended flow rates Fi,int relative to this flow rate. The ideal ratio, naturally always within the window determined by the stoichiometry of the underlying reaction equation, of the intended mass flows Fi,int of the other materials to the flow rate of the prescribed material is known for all customary chemical reactions from the patent and technical literature and can also be determined by engineering calculations known to those skilled in the art.
In the case of a production plant for coupled reactions, the start-up or the setting of relatively large changes in the intended amount produced per unit time therefore requires particular care. In practice, the procedure is to set down the intended mass flow of a starting material or an output material (in the process shown in FIG. 1, for example the first starting material E1) and increasing this from the instantaneous actual value by an essentially linear increase, with the mass flow being increased only slowly. The intended values of the flow rates of the other materials are then set analogously in relation to the flow rate of the prescribed material.
During a “settling-down process” (which will also be explained below with the aid of FIG. 9), more or less large deviations of the actual values from the intended values of the flow rates are unavoidable. Since many mass flows have to be matched to one another in production processes having coupled reactions, the risk of undesirable deviations which in the extreme case could lead to interruption of the process is particularly great. This also applies particularly because increases in the mass flows have hitherto been carried out slowly, namely in many small steps, as a result of which the number of possible error sources is multiplied. In particular, the regulators for the various mass flows can have different regulating deviations over time. This can lead to impermissible deviations in the stoichiometry, i.e. the ratios of the mass flows to one another. For example, a regulator can overshoot, i.e. the actual value exceeds the intended value, while another regulator brings the actual value only very slowly to the intended value, as a result of which the actual value is significantly smaller than the intended value. If such a deviation is not permissible for process-related reasons, the production plant would have to be shut down when the ratios of the actual values of the reactants are outside prescribed intervals.
Although a possible solution is, in the case of relatively large deviations in the desired ratio of the flow rate to the materials fed in relation to one another, which are nevertheless still within particular limit values, to interrupt the further increase in the flow rate of the prescribed material until the system has stabilized again. This procedure has various disadvantages:                The slow increase in flow rate of the output material means a loss of production.        There is an increased requirement for monitoring by operating personnel during such critical phases.        The procedure described is complex and has only a small error tolerance.        
Furthermore, provision of relatively large volumes of intermediate storages for some of the participating materials can be needed. In the example of the production plant shown in FIG. 1, it can, for instance, be necessary, for the purpose of start-up, to keep a relatively large amount of the two coproducts (i.e. the intermediate Z2 and the second output material A2) at the ready in an intermediate storage. This is, at least when it extends over a relatively long period of time, economically disadvantageous and incurs safety risks. This leads to increased capital and maintenance costs. In addition, it is quite possible to conceive of cases in which such a procedure is subjected to limits purely for technical reasons, for instance when one of the coproducts is stable for only a limited time or attacks materials of the intermediate storage on prolonged storage.
However, these particular challenges in relation to the operation of chemical production plants for coupled reactions have hitherto been accorded limited attention in the relevant patent and technical literature. In relation to the production of dinitrotoluene by the coupled reactions nitration of toluene to form mononitrotoluene (e.g. chemical reaction C1 in FIG. 1) and nitration of mononitrotoluene to form dinitrotoluene (e.g. chemical reaction C2 in FIG. 1), the ratios of the individual mass flows have frequently been discussed in the patent literature, but without acknowledging the abovementioned problems in particular. ACS Symposium Series, Vol. 623, chapter 21, “Industrial Nitration of Toluene to Dinitrotoluene”, discloses merely in a quite general way that the inflows of raw materials have to be monitored precisely and emergency shutdowns are provided in particular cases for safety reasons.